This invention relates to electric machines in general and more particularly to electric machines having rotors containing excitation windings which are cooled to a low temperature.
The use of deeply cooled windings, in particular superconducting windings, in electric machines permits a substantial increase of induction in the air gap between the rotating machine part, normally referred to as the rotor, and the stationary machine part, normally referred to as the stator. In particular, it is possible to obtain higher induction with what are known as high field superconductors even without the use of magnetic iron and in a manner permitting the excitation winding to operate almost without losses since very high current densities can be provided in such superconductors. Since no magnetic iron is required, the ampere turns in the stator windings, which are normally conducting, can also be increased considerably while staying within the same machine dimensions. Thus, in a machine using superconducting excitation windings, the ratio of power rating to volume and weight is considerably higher than in a machine of conventional design. Typically the rotor body is a hollow cylinder of non-magnetic material having slots for receiving the excitation winding. The conductors will be, for example, niobium-titanium multi-filiment conductors with a copper or copper nickel matrix for stabilization. The ends of the conductors are connected to current supply and discharge lines which are normally conducting and which supply the required energy to the excitation windings from the outside, i.e. from equipment at room temperature. Thus, it is only the conductors themselves which are cooled to a superconducting temperature. However, to reduce thermal losses due to due to heat influx, cooling is generally provided for the current supply and discharge lines. Surrounding the rotor concentrically is a co-rotating damper winding of copper, for example, which is also kept at a lower temperature of, for example, 80 K. This winding is used both to protect the superconducting excitation winding from alternating magnetic fields and also to reduce the radiated heat.
An a.c. synchronous machine installation having a rotor with superconducting excitation windings which can be maintained in a superconducting state using a cryogenic medium such as liquid helium is described in Swiss Pat. No. 516,250. In the disclosed machine a radiation protection shield is arranged concentrically surrounding the rotor having the superconducting excitation winding. The radiation protection shield is cooled using tubes which carry a coolant such as helium, the temperature of which is below the ambient temperature. The radiation shield is also used as a damper winding in addition to its other functions.
In this reference, as shown on FIG. 4, the machine parts are cooled by a cryogenic medium compressed in a compressor and pre-cooled in a first cooling arrangement. The coolant leaves the cooling arrangement at a temperature of approximately 80 K. and flows to a further cooling stage arranged in the rotor shaft. In this stage the cooling medium is cooled further to a temperature at which superconduction takes place in the excitation windings.
The cooling medium flows from the second cooling stage, now at a temperature of 4 K. for example, inside the rotor to the excitation winding. Part of the cryogenic medium leaving the winding is then used to pre-cool the medium conducted through the second cooling stage after which it is returned to the compressor. A second part of the cooling medium leaving the winding is first led around the second cooling stage and is then used to cool the normal conductors of the current supply and discharge lines. A portion of the cooling medium can be branched off further from the loop between the first cooling device and the second cooling stage and conducted through the tubes attached to the radiation protection shield.
In this prior art cooling arrngement, the current supply and discharge lines are cooled by the same cooling medium which has flowed through the superconducting excitation windings. As a result, cooling of the current supply and discharge lines depends on the cooling of the superconductors of the excitation winding. Since the winding losses of the superconductors can increase to an amount many times their normal value for short periods of time, e.g. in the case of a short circuit, refrigeration must be provided to remove such losses. The accompanying increase of the coolant flow rate through the excitation windings to accommodate these losses may, however, lead to an undesirable cooling of the current supply and discharge leads below that which they are designed for. This extra cooling of the ends of the current supply and discharge lines on the room temperature side may result in them falling below the dew point and may result in ice formation. As a result, a reduction in the dielectric strength of the current supply and discharge lines can occur. Furthermore, large amounts of coolant, sufficient for a peak short circuit, must always be cooled back down to the cryogenic temperature from room temperature. Since such peak short circuits seldomly occur, the cooling procedure of the prior art device is relatively uneconomical.
Thus, the need for an improved cooling device which meets all the various needs of the system but operates economically and without the danger of cooling the supply and discharge lines too much becomes evident.